Micromachined silicon structures frequently are used as sensors and actuators, and for signal processing. For example, in some existing micromachined sensors, micromachined silicon structures are used to detect and measure acceleration, and sometimes use electrostatic or other forces to restore a movable member to its original position. Acceleration may be measured with piezoresistors or capacitors, typically a differential capacitor.
In a capacitive system, the movable member is positioned midway between two plates so that one capacitor is formed by one plate and the member and a second (and equal) capacitor is formed by the second plate and the member. To maximize the capacitance, the member may contain numerous fingers that are interleaved between fingers from the two plates. Various shapes and arrangements of the capacitor plates have been used.
In one such system, the application of a force along a sensitive axis causes the member to move relative to the plates, causing a change in the capacitances of the two capacitors of the differential capacitor, and a signal appears on the member that reflects the amount of acceleration. This signal may be fed back to the member with a negative feedback loop, to create electrostatic forces that will offset the acceleration and maintain the member centered between the plates. An accelerometer based on this principle and a process for fabricating such an accelerometer are described in commonly assigned U.S. Pat. Nos. 5,345,824, 5,326,726, and 5,314,572.
Although this structure is well-suited to measure large accelerations (on the order of 50 g's), it is inadequate to measure accelerations on the order of 5 's, where greater sensitivity is required. In order to obtain this greater sensitivity, it is important to reduce electrostatic forces and parasitic capacitances in the sensing function. The present invention solves this problem as set forth in the remainder of the specification referring to the drawings.